A gravity glitch?

NASA's robot Pioneer 10 and 11 sister spaceships were the first artificial objects to leave the solar system. Both probes explored the outer solar system after being launched from Cape Canaveral, Florida, during the early 1970s.

In 1983, Pioneer 10 was the first terrestrial object to leave the solar system. After three decades of tracking the Pioneers across the solar system, mission control lost radio contact with Pioneer 11 first. Then, Pioneer 10 fell silent in 2003. At the time its radio voice faded away, Pioneer 10 was 7.6 billion miles away from Earth.

Both interplanetary Pioneer probes have since become interstellar space objects. And that's why they are best remembered for the unusual and unearthly communiqu s they carry: attached to the spacecraft platforms are etched, metallic plaques with messages of peace from the planet Earth.

The illustrated Pioneer messages, showing the location of our solar system and Earth as well as creatures that built the probes, are meant for the eyes of extraterrestrials-that is, in the unlikely event alien astronauts encounter the Pioneers adrift in interstellar space.

Similar messages are onboard the Voyager 1 and 2 spacecraft which has already passed Pioneer 10 and 11 in deep space.

Those of us who remember the exciting Pioneer 10 and 11 flybys of the giant planets Jupiter and Saturn during the 1970s may not be aware that Pioneer 10 left the science community with a space mystery to solve-a mystery now known the Pioneer Anomaly.

To understand the complex mathematics behind the Pioneer Anomaly would require a deep study of gravitational and Doppler physics, but the easiest way to explain the Pioneer mystery is to describe it simply this way:

When a spacecraft leaves the solar system-in our case, Pioneer 11- it slows down. It's not supposed to that according to the way physics is understood today. So, before Pioneer 11 exited our solar system in 1983, physicists and space mission planners expected the opposite effect.

Thanks to radio tracking data received on Earth by the ever receding Pioneer 11, the anomaly became a fascinating problem for space researchers to solve.

Physicists attribute the Pioneer slowdown to the gravitational effects of the Sun. While common sense tells me the effects of the Sun's gravity on a spacecraft leaving our solar system should grow weaker (not stronger), the opposite seems to be the case. As a result of the gravity field change, the Pioneers are not to be found in the spot in deep space they're supposed to be at-they are elsewhere.

According to NASA-JPL's John Anderson, who has been studying this anomaly since the 1980s, "Something strange is happening in the outer reaches of our solar system. The Pioneer 10 and 11 spacecraft are not where they are supposed to be. These missions, launched in 1972 and 1973, have covered hundreds of millions of kilometers, heading toward the edge of our solar system. But something is holding them back. Each year, they fall behind in their projected travel by about 5,000 kilometers (3,000 miles)."

Anderson and his team of researchers haven't found figured it out-yet.

"No spacecraft behavior or previously unknown property of the outer solar system can explain the deceleration of the Pioneer spacecraft," says the Planetary Society's website about the problem. "Scientists are being forced to consider the unthinkable: something may be wrong with our understanding of the laws of physics. An important line of inquiry will be to study mounds of Doppler (velocity) data and spacecraft status data (like temperatures) that have been unavailable to researchers-but that is about to change."

Anderson said "We have now recovered data covering nearly 30 years to help solve the mystery, in addition to recovering never before analyzed spacecraft status data that will be valuable in assessing possible spacecraft influences on the anomaly. Analysis of the recovered data is now in progress."

What's in the Sky: This Saturday and Sunday, look for dull-red Mars near the horizon in the west-southwest after sunset; the Red Planet is 4 degrees right of bright Venus and 2 degrees right of Spica.

Lou Varricchio, M.Sc., was a science writer at the NASA Ames Research Center and a part of the NASA-Raytheon team supporting work on the X-33 aerospace vehicle and related work there in the early 2000s.